Shallow cumulus cloud feedback in large eddy simulations – bridging the gap to storm-resolving models

Jule Radtke,Thorsten Mauritsen,Cathy Hohenegger

doi:10.5194/acp-21-3275-2021

Abstract

Abstract. The response of shallow trade cumulus clouds to global warming is a leading source of uncertainty in projections of the Earth's changing climate. A setup based on the Rain In Cumulus over the Ocean field campaign is used to simulate a shallow trade wind cumulus field with the Icosahedral Nonhydrostatic Large Eddy Model in a control and a perturbed 4 K warmer climate, while degrading horizontal resolution from 100 m to 5 km. As the resolution is coarsened, the base-state cloud fraction increases substantially, especially near cloud base, lateral mixing is weaker, and cloud tops reach higher. Nevertheless, the overall vertical structure of the cloud layer is surprisingly robust across resolutions. In a warmer climate, cloud cover reduces, alone constituting a positive shortwave cloud feedback: the strength correlates with the amount of base-state cloud fraction and thus is stronger at coarser resolutions. Cloud thickening, resulting from more water vapour availability for condensation in a warmer climate, acts as a compensating feedback, but unlike the cloud cover reduction it is largely resolution independent. Therefore, refining the resolution leads to convergence to a near-zero shallow cumulus feedback. This dependence holds in experiments with enhanced realism including precipitation processes or warming along a moist adiabat instead of uniform warming. Insofar as these findings carry over to other models, they suggest that storm-resolving models may exaggerate the trade wind cumulus cloud feedback.

Highlights

How shallow cumulus clouds respond to global warming has been recognized as a critical source of uncertainty to processor model-based estimates and interpretations of the Earth’s changing climate (Bony and Dufresne, 2005; Vial et al, 2013; Zelinka et al, 2020; Flynn and Mauritsen, 2020; Sherwood et al, 2020)
In models probed in the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the low-level cloud feedback varies between 0.16 and 0.94 W m−2, with most of the spread coming from the low-cloud amount feedback, the latter with values ranging between −0.09 and 0.63 W m−2 (Boucher et al, 2013; Zelinka et al, 2016)
Regardless, though, we find the same dependency on resolution of how shallow cumulus cloud coverage responds to warming in both precipitating and non-precipitating simulations

Summary

Introduction

How shallow cumulus clouds respond to global warming has been recognized as a critical source of uncertainty to processor model-based estimates and interpretations of the Earth’s changing climate (Bony and Dufresne, 2005; Vial et al, 2013; Zelinka et al, 2020; Flynn and Mauritsen, 2020; Sherwood et al, 2020). Shallow cumulus clouds are observed in the tropical trade wind region and often called trade wind cumuli, even if they appear in most regions on Earth Due to their widespread occurrence over the world’s oceans, shallow cumuli are, though small in size, crucial to the Earth’s radiative balance and act to cool the Earth by reflecting shortwave radiation (Hartmann et al, 1992). Their response to global warming is important for the global-mean cloud feedback. In models probed in the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the low-level cloud feedback varies between 0.16 and 0.94 W m−2, with most of the spread coming from the low-cloud amount feedback, the latter with values ranging between −0.09 and 0.63 W m−2 (Boucher et al, 2013; Zelinka et al, 2016)

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Conclusion